1. Field of the Invention
The present invention relates to a linear actuator, especially for use in a telescopic lifting column.
2. The Prior Art
Height-adjustable desks, also known as sitting/standing tables, comprise a frame with an upper frame and a lower frame. The upper frame typically consists of two transverse members connected to two parallel sections. A table top is carried by and secured to the upper frame. The lower frame comprises a telescopic lifting column, designed as a table leg, at each side, the lifting columns being mutually connected by a cross member. A foot, typically elongated, faces across the table and is secured to the lower end of the telescopic columns, The telescopic columns are driven by a linear actuator comprising a box-shaped housing containing a reversible electric motor and a transmission. Secured to the bottom of the box-shaped housing is a linear movable activation element consisting of a spindle unit, connected to the transmission through a hole in the bottom of the housing. The housings of the linear actuators face along the table and towards each other. The upper frame of the table is secured to the lower frame by means of the housings of the linear actuators, which are located between the two parallel beams of the upper frame and are secured by screws in the side of the housings. Likewise, the transverse members of the upper frame are usually also secured by screws in the end of the housing of the actuator. Such construction is for instance disclosed in EP 1 079 511 A1, Linak A/S. The cross member, which is secured to the top of a fixed member of the telescopic lifting columns, contributes significantly to the stability of the table in the longitudinal direction.
EP 1 274 330 B1, Okin Gesellschaft für Antriebstechnik mbH Co. KG, discloses an example of a lifting column where the spindle unit is placed at one end of the housing so that the housing is located perpendicularly to the telescopic guide. For cost and production reasons, but also for weight reasons, the two parallel beams are gradually replaced with beam sections. As these beam sections are not continuous, they do not contribute as much to the stability in the longitudinal direction as the through-going beams. For appearance reasons it is also desired that the housing of the linear actuator is as small as possible, so that it may be hidden under the table top. In order to construct the housing as low as possible “flat” electric motors are among other things used, cf. e.g. WO 2004/100632 A1 Linak A/S. This small and low housing also contributes to reducing the stability in the cross direction of the table. For that reason the cross member is particularly important for the stability. On the other hand, there is a distinct desire to eliminate the cross member both visually, due to cost and construction, but also for safety reasons. The latter is due to the risk of squeezing between the underside of the upper frame/the table top and the cross member. By removing the cross member problems with the stability however occur in the longitudinal direction. This is sought solved with various types of angular reinforcements in the angle between the underside of the housing and the side of the telescopic guide in the lifting column. WO 2006/053559 A2 Linak A/S discloses an example of this where a triangular plate element is welded on. This is however only possible in case of a lifting column of the type up-side-down, i.e., where the outmost member of the telescopic guide is secured to the underside of the housing.
The purpose of the invention is to provide a solution to the outlined problem with strengthening in the longitudinal direction of the table, in which the solution is not price-raising or noticeable price-raising and where the solution does not disturb the appearance of the table.